1. Field of the Invention
The present invention relates to a rolling-contact shaft with a joint claw, particularly a steel-made rolling-contact shaft with a joint claw at an end, and having a portion of the outer cylindrical surface functioning as the raceway of a rolling element of a bearing.
2. Description of the Background Art
There are cases where a rolling-contact shaft with a joint claw is employed as a shaft component to transmit momentum. The rolling-contact shaft includes a claw formed at an end, coupled to another adjacent shaft, and has a portion of the outer cylindrical surface functioning as the raceway of a rolling element of a bearing. For example, a rolling-contact shaft employed in a brake actuator may have a portion of the outer cylindrical surface surrounded by a needle roller qualified as a rolling element arranged in contact with a portion thereof, and supported rotatably by a needle roller bearing absent of an inner ring. In other words, the rolling-contact shaft serves a function equivalent to an inner ring, and a portion of the outer cylindrical surface thereof functions as a raceway of a needle roller qualified as a rolling element.
When such a rolling-contact shaft is employed, rolling contact fatigue life is required at the portion of the outer cylindrical surface thereof. To this end, medium carbon steel for machine structural use (such as S53C of JIS: Japanese Industrial Standard) is employed for the raw material of the rolling-contact shaft, taking into consideration the configuration of the shaft, the manufacturing cost, and the like. Additionally, the region where hardness is required such as the region corresponding to the raceway is subjected to high frequency induction heat treatment (induction hardening). In the case where flaking will occur at the rolling-contact shaft earlier than at the bearing that supports the rolling-contact shaft, high carbon chromium bearing steel (such as SUJ2 of the JIS) is employed for the raw material in order to improve the rolling contact fatigue life of the rolling-contact shaft, and the rolling-contact shaft is subjected to high-frequency induction heat treatment (induction hardening), bright heat treatment, carbonitriding heat treatment (carbonitriding quenching), and the like. Further, manganese steel for machine structural use (such as SMn420 of the JIS), manganese chromium steel (such as SMnC420 of the JIS), chromium steel (such as SCr420 of the JIS), chromium molybdenum steel (such as SCM415 of the JIS), nickel chromium steel (such as SNC415 of the JIS), nickel chromium molybdenum steel (SNCM420 of the JIS) or the like may be employed, and subjected to high frequency induction heat treatment (induction hardening), carburizing heat treatment including high density carburizing (carburizing quenching), carbonitriding heat treatment (carbonitriding quenching), or the like.
In recent years, there is the demand for reduction in size, weight, and space of components such as brake actuators. In view of such demand, the approach of forming a joint claw at an end of the rolling-contact shaft employed as a component for a brake actuator and the like, directed to reducing space, has been employed (refer to Japanese Patent Laying-Open Nos. 06-247253 and 06-286577). Such a rolling-contact shaft with a joint claw allows direct connection between shafts without having to use a coupling that is the general member to establish connection between shafts, and also contributes to reducing the number of employed components to suppress the manufacturing cost as well as to reduce space.
The demand for reduction in size, weight, and space for components such as brake actuators also induces the demand for reduction in size, weight and space for bearings employed in such components. For example, in the case where the needle roller bearing is reduced in size in a rolling-contact shaft supported rotatably by the aforementioned needle roller bearing absent of an inner ring, the rolling-contact shaft serving a function equivalent to an inner ring is also reduced in size and has a smaller outer diameter. This means that, in the case where the same load is applied, the contact pressure between the needle roller and the outer cylindrical surface of the rolling-contact shaft will increase. As a result, the rolling contact fatigue life must be improved at a region of that outer cylindrical surface of the rolling-contact shaft. When the rolling-contact shaft is connected to another member through its joint claw in the case where the rolling-contact shaft is reduced in size, the torsional stress loaded on the claw is increased, necessitating an improvement in the torsional strength of the claw portion.
In order to improve the rolling contact fatigue life of the rolling-contact shaft, various measures are possible such as increasing the hardness of the raceway, increasing the alloy contents such as Cr contained in the steel raw material, applying carburizing or carbonitriding heat treatment, or the like. It is to be noted that such approaches may not necessarily improve the static fracture strength such as the torsional strength of the claw, and, in some cases, may degrade the strength instead. It was therefore difficult to achieve an improvement in both the rolling contact fatigue life at the raceway of the rolling-contact shaft and also the torsional strength of the claw region through the conventional approaches of improving the rolling contact fatigue life set forth above.